1. Field of the Invention
This invention relates to ignition systems for internal combustion piston engines.
2. Description of Prior Art
Piston engines for aircraft often have two independent ignition systems so an engine will continue to operate if one ignition system fails. This includes two spark plugs per cylinder, with a separate alternator, coils, distribution components, and timing sensing components for each set of plugs.
A piston engine in which both cylinders in each pair of opposed cylinders fire simultaneously is called xe2x80x9ctruly-opposedxe2x80x9d. A new design for a 12-cylinder 4-cycle truly opposed piston engine was developed by William S. Nagel and Philip L. Reid, and is the subject of a separate patent application. Chris Watson is a consultant to the inventors of that engine, and is the inventor of the ignition system described herein.
The preferred crankshaft and timing table for the Nagel/Reid engine is shown in FIGS. 1-3 herein. This engine uses a dual Ignition transformer mounted above each cylinder on the cylinder head. Each dual ignition transformer has two secondary windings and provides two spark output leads. The first spark lead fires one of two sparkplugs in the cylinder on which the coil is mounted, and the second spark plug lead simultaneously fires one of two sparkplugs in the opposed cylinder on the opposite side of the engine. Thus, the second spark output lead must cross over the engine.
A conventional truly-opposed engine fires the cylinders on multiples of 1440 degrees divided by the number of cylinders. For example, in a 12-cylinder engine, two opposed cylinders fire at each 120 degrees of engine rotation. For practical reasons, ignition timing for such an engine is normally obtained by multiply sensing a timing mark on a disk attached to the crankshaft. A 4-cycle engine rotates 720 degrees for each complete 4-stroke cycle, and timing signals are distributed over 720 degrees. A timing disk rotating with the crankshaft also rotates 720 degrees per full cycle. Thus, timing signals for the first 360 degrees of the ignition sequence overlay timing signals for the second 360 degrees. For example, the 0-degree signal is the same as the 360-degree signal, even though a different pair of cylinders is to be fired at 0 degrees and at 360 degrees.
The ambiguity caused by this timing overlay has been previously handled by sending the same distributor output voltage to two coils at each rotation of the timing mark. For example, the pair of cylinders to be fired at 0 degrees, and the pair of cylinders to be fired at 360 degrees are both sparked at both times. This provides a spark twice as often as needed in each cylinder. However, at each spark time only one of the two pairs of cylinders is on the compression stroke and is fired by the spark. The second pair of cylinders is either on the end of the exhaust stroke or the beginning of the intake stroke, and the spark is wasted. When the second pair of cylinders is on the compression stroke, both pairs of cylinders are sparked again, firing the second pair of cylinders. This solution avoids the need to distinguish between timing marks 360 degrees apart in the 720-degree timing cycle. It also halves the number of distributor outputs that would otherwise be needed for a separate spark to each pair of cylinders.
However, this convenient solution is not available in the preferred Nagel/Reid engine, because Nagel/Reid discovered that providing an alternating positive and negative offset to the timing sequence reduces harmonic vibrations in the crankshaft. Instead of using a spark every 120 degrees, the Nagel/Reid engine uses timing shown in FIG. 2 as an example. Other timing variations may be used in this concept. The essence is that positive and negative offsets balance, so the average timing separation between pairs of cylinders is still 1440 degrees divided by the number of cylinders. It doesn""t matter if the first offset is positive or negative.
In an engine with timing offsets, the piston cranks are also offset to match, as shown in FIGS. 1 and 3. The spark is still provided at the same optimum point in the compression stroke of each cylinder. For ignition timing with offsets, the ignition distributor must determine which pair of cylinders is in compression. The present invention provides a way of making this distinction.
An objective of the present invention is provision of ignition system timing for a truly opposed 4-cycle piston engine in which the ignition timing sequence is in steps of 1440 degrees divided by the number of cylinders plus an alternating positive/negative offset. A second objective is to obtain the ignition timing by sensing marks on a timing disk rotating at the speed of the crankshaft. A third objective is to use a low voltage electronic distributor, ignition coils mounted on the cylinder heads, and spark plug leads that do not cross over the engine. A fourth objective is to minimize the number of transformers and the number of wires between the distributor and the transformers, making dual redundant ignition systems practical.
These objectives are achieved by:
a) providing a cam sensor disk connected to a camshaft of the engine, the cam sensor disk having a circumference, with a first half of the circumference differentiated from the second half for sensing;
b) providing an ignition timing disk connected to the crankshaft of the engine, the timing disk having a circumference with a timing mark;
c) providing sensors for sensing the timing mark, the sensors located in a sequence around the circumference of the timing disk, starting at 0 degrees, in steps of 1440 degrees divided by the number of cylinders of the engine plus an offset that alternates between positive and negative;
d) providing an electronic distributor with an output lead for each two cylinders on the same side of the engine that are fired by respective two adjacent sensors separated by the magnitude of the offset on the timing disk; and,
e) on each 360-degree rotation of the timing disk, selecting which of said respective two adjacent sensors to enable for said each two cylinders based on the position of the cam sensor disk;
whereby the spark timing for each cylinder alternates between two timings, a first timing to fire the cylinder, and a second timing to fire a second cylinder on the same side of the engine using the same distributor output lead.